Download Geiger-Mueller tube

The Geiger-Mueller tube
A diagram of a Geiger-Mueller tube is shown in Figure 1.
A typical Geiger tube can
detect separate particles as
long as they arrive more than
200 microseconds apart and
therefore it has a maximum
count rate of 5000 counts per
second.
This device is basically a gasfilled cold-cathode diode, in
which the anode is a metal rod
fixed along the axis of a
cylindrical cathode.
low pressure neon gas
+450 V
anode
0V
end window
Figure 1
The anode should be thin, so that an intense electric field is produced near it when a
potential is connected between the anode and cathode. The end of a tube is closed by a
‘window’, the thickness of which varies from tube to tube depending on the type of radiation
it is designed to detect.
The thickness of the end window is quoted in mg cm-2 for alpha-particles it is about 2, for
beta-particles about 25 and for gamma-rays many hundred. The tube contains neon at about
10 cm of mercury pressure, and a potential of about 450 V is applied between anode and
cathode.
When a particle enters through the end
window ions are produced in the gas.
The positive ions travel towards the cathode
while the electrons move towards the anode
(Figure 2). As they move they produce further
ions by collisions, a process known as secondary ionisation, and an avalanche of ions
reaches the detecting electrodes. For an
electron about 108 ions are produced in a few
microseconds. This pulse is amplified in an
external circuit and detected as either a meter
reading or a sound. To prevent continuous
secondary ionisation a little bromine gas is
added to the tube, acting as a ‘quenching
agent’ and absorbing the kinetic energy of the
positive ions.
If the characteristics of the Geiger tube (the
anode voltage related to the count rate) are
recorded as shown in Figure 3, it can be seen
that the tube should be operated in the socalled plateau region. In this area a small
change of anode potential will have little effect
on the count rate.
radioactive particle
Figure 2
anode
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The Geiger tube may be fitted to a variety of
detectors for investigating the activity of a
radioactive source:
(a) a scaler - this device simply records the total
number of pulses;
(b) a speaker and an amplifier - this will give an
audible signal that becomes a continuous
crackle when the activity is high;
(c) a ratemeter - this actually records the count
rate (dN/dt) and the output may be fed to a
meter or to a storage facility.
If a Geiger tube with a thin end window is used
in a darkened room, flashes of light may be
observed in the tube when it is used to detect
particles from an alpha source.
Count rate
Anode voltage
Figure 3
Problem
The energy of an alpha-particle emitted by polonium-210 is 3.9 MeV, and its range in air at
standard temperature and pressure (s.t.p.) is 35 mm. As it passes through the air the alpha
particle produces ion pairs, the energy required to produce each ion pair being about 30 eV.
(a) Estimate the number of ion pairs formed per mm of path
(b) The ionisation per mm increases towards the end of the path. Suggest a reason for this.
If the pressure was reduced to one-hundredth of the original value, how would this affect the
number of ions per mm? Explain your answer.
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